Aristonectes (meaning "best swimmer") is an extinct genus of large elasmosaurid plesiosaurs that lived during the Maastrichtian stage of the Late Cretaceous. Two species are known, A. parvidens and A. quiriquinensis, whose fossil remains were discovered in what are now Patagonia and Antarctica. Throughout the 20th century, Aristonectes was a difficult animal for scientists to analyze due to poor fossil preparation, its relationships to other genera were uncertain. After subsequent revisions and discoveries carried out from the beginning of the 21st century, Aristonectes is now recognised as the type genus of the subfamily Aristonectinae, a lineage of elasmosaurids characterized by an enlarged skull and a reduced length of the neck.
Measuring more than long, Aristonectes is a notably imposing plesiosaur. A referred specimen discovered in Antarctica has an estimated size of more than long, which would make this genus one of the largest known plesiosaurs. The paddle-like limbs of Aristonectes are unusually large for an elasmosaurid, reaching almost in length, suggesting a total wingspan of around for the animal. The skull is ogival-shaped and is flattened, possessing sharp forward-facing teeth. According to estimates, 13 teeth would have been present in each premaxilla, 50 teeth in the maxilla and 50 to 63 or more teeth in the lower jaw.
According to its morphology, mainly cranial, Aristonectes fed by mixing prey and sediment in the benthic zones, like the modern gray whale. As in other plesiosaurs, gastroliths (stomach stones) would have been used by Aristonectes either to help digest its food or to reduce excess buoyancy, although there is little support for the latter hypothesis. According to its geographical distribution in the fossil record, Aristonectes would have regularly migrated between Patagonia and Antarctica.
Research history A. parvidens
The first Aristonectes fossil was discovered long before the genus was named by Ángel Cabrera in 1941. In 1848, the Franco-Chilean naturalist Claude Gay described the first known plesiosaur from South America, Plesiosaurus chilensis, on the basis of a single caudal vertebra discovered in Quiriquina Island, in Concepción Province, Chile. In a book published in 1889, Richard Lydekker placed this species in the genus Cimoliosaurus. In 1895, Wilhelm Deecke moved it into the pliosaurid genus Pliosaurus as Pliosaurus chilensis, and referred other fossils that have been discovered in the same locality to this species. Among these fossils are some vertebrae, a femur, fragments of an ischium, and ribs. In 1918, the Russian paleontologist moved it into the related genus Elasmosaurus, as Elasmosaurus chilensis. In 1941, Cabrera described a different specimen from Chubut Province, Argentina as the new genus and species Aristonectes parvidens. In the same publication, he listed Elasmosaurus chilensis as "Plesiosaurus" chilensis, expressing uncertainty regarding its affinities. In 1949, Edwin H. Colbert found that the holotype specimen of "P." chilensis (the caudal vertebra originally described by Gay) belongs to a pliosauroid, but also was uncertain about its generic placement. Also in the article, he considers that the other fossil material previously attributed to "P." chilensis could come from elasmosaurids. In an analysis published in 2013 by José P. O'Gorman and colleagues, the holotype specimen of "P." chilensis was recognized as potentially belonging to A. parvidens. As this specimen merely consists of a single caudal vertebra, it was referred to as A. cf. parvidens to indicate uncertainty in its assignment.
The first specimen formally identified as A. parvidens was collected by Cristian S. Petersen in collaboration with local resident Victor Saldivia, in the , near the municipality of Paso del Sapo in Chubut Province, Argentina. The specimen was discovered in the Lefipán Formation and was dated to the Maastrichtian. In September 1940, Pablo Groeber sent this specimen to the Museo de La Plata as a donation for the Dirección de Minas y Geología del Ministerio de Agricultura (Directorate of Mines and Geology of the Ministry of Agriculture). A vertebra and incomplete phalanges from the same region had previously been donated to the Museo de La Plata by Mario Reguiló, and were subsequently referred to the holotype, catalogued as MLP 40-XI-14-6, as they most likely came from the same specimen. The holotype consists of a partial skull attached to the mandible, the atlas and axis, 21 cervical vertebrae (including 16 anterior cervicals which are articulated), 8 caudal vertebrae and an incomplete left forelimb. The specimen was interpreted as an adult, or even an elderly individual. Cabrera described the animal as Aristonectes parvidens in his article published in 1941 by the museum that houses the fossil material. The genus name Aristonectes comes from the Ancient Greek words ἄριστος (áristos, "best", "superior") and νηκτός (nêktós, "swimmer"), and may be translated as "best swimmer", so named to indicate the best preserved specimen of plesiosaur from Argentina known at that time. The specific epithet parvidens means "small teeth", in reference to the rather small size of the tooth sockets.
Since the unknown parts of the original specimen were heavily reconstructed, the precise anatomy of Aristonectes remained uncertain throughout the 20th century. In 2003, Argentine paleontologist Zulma Gasparini and colleagues re-prepared the holotype skull along with the atlas and axis, in an attempt to clarify its anatomy. In a doctoral thesis published in 2013, O'Gorman informally assigns several caudal vertebrae to A. parvidens. In 2016, the same author revised the holotype again.
A. quiriquinensis
The first discovery of the second known species, A. quiriquinensis, dates back to the late 1950s, when Argentinian paleontologist Rodolfo Casamiquela discovered a partial skeleton in Las Tablas Bay, located north of Quiriquina Island, Chile. The specimen, cataloged as SGO.PV.260, was found in Maastrichtian beds of the Quiriquina Formation; initially, five vertebrae, a humerus fragment, and the distal end of a limb were visible. The hardness of the rocks containing the fossils made subsequent preparation difficult. The preparation uncovered additional body parts, with the specimen including all four limbs, a posterior portion of the neck, most of the trunk, and a complete tail. It is interpreted as a juvenile. This specimen was first analyzed in 2012 and identified as an indeterminate aristonectine, before being assigned to the genus Aristonectes in 2013.
In 2001, Chilean paleontologist Mario E. Suárez collected a partial skull, mandibular fragments and 12 anterior cervical vertebrae from a beach near , located north of Tomé. This material was described in the following year and attributed to Aristonectes, but identification as a separate species was then impossible due to lack of preparation. In early 2009, a second excavation was carried out independently at the same site by a team from the University of Concepción of Chile and the University of Heidelberg of Germany. This excavation recovered 119 blocks of sandstone, most of them with bone material, but some were degraded by the periodic immersion of sea water that caused brittle surfaces on the most delicate parts. Additionally, several anatomical contacts were lost as blocks of sandstone were cut from the beach using a rock saw at low tide. The precise location of the find and the taphonomic distribution of the fossils show that they belong to the same specimen that was discovered in 2001. The bones were sent to the Chilean National Museum of Natural History for the preparation and scientific analysis. The completely prepared fossil consists of the skull, the axis and the atlas, 12 anterior cervical vertebrae, 23 middle to posterior cervical vertebrae, most of the trunk, the two almost complete forelimbs and a significant part of the right hind limb. The specimen was likely a young adult, and is cataloged as SGO.PV.957. In 2014, Rodrigo Otero and colleagues made it the holotype of the new species A. quiriquinensis; these researchers also referred the earlier specimen, SGO.PV.260, to this species. The specific epithet is a reference to the formation of Quiriquina, which is the type locality of this taxon.
In 2015, two other specimens from Quiriquina Island were referred to A. quiriquinensis. These are composed of a complete left femur and a proximal part of a humerus, cataloged respectively as SGO.PV.135 and SGO.PV.169, which were previously referred to the now dubious genus Mauisaurus. In 2018, the holotype specimen was redescribed after further preparation. In the same publication, another specimen, SGO.PV.94, consisting of a series of anterior caudal vertebrae, was also referred to A. quiriquinensis.
Description
Aristonectes is a plesiosaur that had been particularly difficult for scientists to analyze because of its incompleteness. Its relationships remained unclear until more recent research carried out on A. parvidens and the discovery of the second species A. quiriquinensis have made it possible to redescribe its anatomy and classify it among the elasmosaurids. However, anatomical comparisons between A. parvidens and A. quiriquinensis are still limited to elements that are known from both species. Aristonectes is a large representative of the plesiosauroids, the holotype of A. quiriquinensis having an estimated total body length of about or more than long and body mass of nearly . A referred specimen of Aristonectes discovered in the López de Bertodano Formation in Antarctica, cataloged as MLP 89-III-3-1, is considered to be one of the largest and heaviest plesiosaurs identified to date, having an estimated size between long and a body mass of .
Skull In the two known species, the skull of Aristonectes is flattened and ogive-shaped. The size of the skull varies very slightly between the two species, that of the holotype of A. quiriquinensis having a proposed size between , while from the holotype of A. parvidens measures . In A. quiriquinensis the squamosal bones extend well posterior to the occipital condyle. The pterygoid bones are large and project posterior to the neurocranium. The posterior extensions of the pterygoid bones are surrounding the axis and the atlas, which reduces the space between the skull and the vertebrae, and would therefore limit lateral movement at their joints. Around 13 teeth were present in the premaxilla and 50 in the maxilla. Among the unique characters of the skull of A. quiriquinensis is the presence of a mental boss (a ridge along the mandibular symphysis), a feature not observed in the holotype of A. parvidens.
The mandibular symphysis of Aristonectes is short. The dentary bones, although being partially preserved in both species, would have an estimated total of 50 teeth in A. quiriquinensis and 63 or more in A. parvidens. The mandibular symphysis of the dentary bones in A. quiriquinensis is comparatively thicker and lacks the deep groove seen in ventral view in A. parvidens. The most posterior teeth of A. parvidens were about rostrally from the coronoid process. In A. quiriquinensis, there are no tooth sockets present, although of the mandible is preserved rostral to the coronoid process. Therefore, it is likely that A. quiriquinensis would have fewer teeth than A. parvidens. In A. parvidens, the labial contact between the angular bone and the surangular, anterior to the glenoid fossa, bears a deep cavity which is not present in A. quiriquinensis.